The goals of the proposed research are to elucidate the mechanisms of aminoglycoside ototoxicity and to develop a rational preventive treatment. Aminoglycoside antibiotics have been a frontline in systemic therapy for serious Gram-negative infections for nearly five decades although their ototoxic and nephrotoxic side effects were recognized soon after the introduction of streptomycin in the 1940s. To date, the mechanisms of toxicity have not been established and the absence of protective therapeutic measures continues to complicate the clinical application of these drugs. Previous attempts to define mechanisms of action and to design antidotes were based on the belief that aminoglycosides do not undergo significant metabolism in the body. Our novel hypothesis which is based on recent experimental evidence that a toxic metabolite exists, contradicts this dogma and enables us to address questions that previous hypotheses left unanswered. Our aim is to characterize the metabolite and elucidate its contribution to the mechanisms of ototoxicity. The goals of the proposal will be accomplished by experiments on 1) chemical analysis of the metabolite and the enzymatic mechanisms of its formation; 2) mechanisms of the toxic action of the metabolite at the cellular and molecular level; 3) mechanisms of organ-specific toxicity; and 4) prevention or amelioration of aminoglycoside ototoxicity. These specific questions will be addressed with well-established biochemical, analytical and physicochemical techniques. The elucidation of the mechanism of aminoglycoside toxicity will provide a scientific rationale with far-reaching implications for the prevention or amelioration of adverse effects of a family of drugs whose primary efficacy is unquestioned.